"One would hope that the maturity of years and an appreciation of the deep-seated pain still blighting the lives of the families, as well as being true to one's conscience, would encourage anyone with even the slightest piece of information to come forward."

Crowds gathered on Saturday at a spot near Kells, Co Meath, where one of the victims is thought to have been secretly buried.

Newlywed Brendan Megraw, 23, vanished from west Belfast in 1978.

His brother Kieran was one of a number of victims' relatives at the ceremony.

Earlier, Dr Smith prayed with Phil McKee at nearby Wilkinstown where it is thought her brother Kevin and Seamus Wright who disappeared in 1972 are buried.

"The horror and barbarity of what they suffered caused the deepest of pain which was only added to by the callous manner in which their bodies were disposed," Dr Smith said.

"Not having a focus for grieving gives rise to a permanent nightmare."

The bishop also paid tribute to the Independent Commission for the Location of Victims Remains, which was jointly established by the British and Irish governments in 1999 to co-ordinate searches for the bodies of the Disappeared.

The joint commissioners Sir Kenneth Bloomfield and Frank Murray attended the ceremony, which was organised by the Wave Trauma Centre - a charity that supports those bereaved or injured during the conflict.

Mr Murray insisted that no one who has come forward so far with information about the Disappeared has been prosecuted or arrested.

"All searches are information-led and we do not carry out speculative searches."

Between 1972 and 2003, 17 people have been acknowledged as "disappeared" during the conflict, having been kidnapped, murdered and disposed of in unmarked graves by republicans.

The remains of 10 bodies have been recovered.

Wave trauma group chief executive Sandra Peake said the families were heartened by the number of local people who attended the ceremony.

"It really helps give them the strength and resolve to continue down this painful road," she said.

Mr Megraw was described by his family as motorbike mad - fixing, cleaning and racing them. Politics was one of his pet hates.

The IRA claimed he confessed to being a British provocateur and Military Reaction Force undercover agent in 1978.

Others missing include Joe Lynskey, a former Cistercian monk from West Belfast, and Captain Robert Nairac of the SAS.

In 1999 the British and Irish governments jointly established the Independent Commission for the Location of Victims' Remains (ICLVR). Sinn Fein president Gerry Adams led appeals for information in finding the bodies.

The ICLVR co-ordinates the searches using the latest forensic techniques.

Anyone with information is asked to contact the commission, which treats information as confidential.